1. Field of the Invention
The present invention relates to a scanner motor rotation control technique in an image forming apparatus.
2. Description of the Related Art
Color misregistration undesirably leads directly to degradation of the image quality in image forming apparatuses such as a color copying machine and color printer. The color misregistration is reduced by, for example, adjusting the position of the reflecting mirror in the optical path or the image write timing in accordance with the result obtained by reading a pattern formed on an intermediate transfer belt or convey belt.
However, this method alone cannot reduce expansion and contraction (distortion) of an image in the subscanning direction caused by periodic fluctuations of the speed due to the eccentricity (irregularity) of the photosensitive drum, thickness variations of the belt, the irregularity of the driving system, and the like. The expansion and contraction of an image also result in color misregistration. Note that the subscanning direction is rotation direction of the photosensitive drum.
To cancel periodic fluctuations of the photosensitive drum, belt, and the like, there is proposed a method of sequentially changing the rotation speed of the scanner motor (Japanese Patent Laid-Open No. 10-003188). More specifically, when the speeds of the photosensitive drum, belt, and the like are high, the scanner motor accelerates. When the speeds of the photosensitive drum, belt, and the like are low, the scanner motor decelerates. This can reduce expansion and contraction of an image in the subscanning direction.
However, the actual tracking characteristics of the scanner motor to a change of the target rotation speed are different between scanner motors. Even a single scanner motor gradually degrades its tracking characteristics with time in use.
If the scanner motor cannot track a change of the target rotation speed, neither expansion nor contraction of an image in the subscanning direction can be reduced.